linux-kernel - Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl

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Date:   Thu, 16 Feb 2017 11:17:58 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Juri Lelli <juri.lelli@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, peterz@...radead.org,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...il.com, kernel-team@....com,
        Luca Abeni <luca.abeni@...tannapisa.it>
Subject: Re: [PATCH] sched/deadline: Remove unnecessary condition in
 push_dl_task()

On Wed, Feb 15, 2017 at 02:45:03PM +0000, Juri Lelli wrote:
> On 15/02/17 09:25, Steven Rostedt wrote:
> > On Wed, 15 Feb 2017 10:47:49 +0000
> > Juri Lelli <juri.lelli@....com> wrote:
> > 
> > > [+Steve, Luca]
> > > 
> > > Hi,
> > > 
> > > On 15/02/17 14:11, Byungchul Park wrote:
> > > > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > > > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > > > task == next_task. Remove a redundant condition and make code simpler.
> > > > 
> > > > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > > > ---
> > > >  kernel/sched/deadline.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > > index 27737f3..ad8d577 100644
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> > > >  		 * then possible that next_task has migrated.
> > > >  		 */
> > > >  		task = pick_next_pushable_dl_task(rq);
> > > > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > > > +		if (task == next_task) {  
> > > 
> > > Seems a sensible optimization to me. Actually, we are doing the same for
> > > rt.c; Steve, Peter, do you think we should optimize that as well?
> > > 
> > 
> > Are we doing the same for push_rt_task()? I don't see it, and I don't
> > see it in tip/sched/core either. What I have is:
> > 
> >   if (task_cpu(next_task) == rq->cpu && task == next_task) {
> 
> Sorry, bad wording on my side. I meant the are currently checking the
> same conditions both for DL and for RT, and we should probably optimize
> RT as well if we are going to take this patch.
> 
> > 
> > But that said, I believe this patch is correct, and we should change
> > rt.c as well.
> > 
> 
> That's what I meant. :)
> 
> > 
> >   task = pick_next_pushable_dl_task(rq);
> > 
> > Which has:
> > 
> >   BUG_ON(rq->cpu != task_cpu(task))
> > 
> > when it returns a task other than NULL. Which means that task_cpu(task)
> > must be rq->cpu. Then if task == next_task, then task_cpu(next_task)
> > must be rq->cpu as well.
> 
> Right.
> 
> > 
> > Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> > 
> 
> You can also add mine
> 
> Reviewed-by: Juri Lelli <juri.lelli@....com>

Juri and steven, thank you very much for reviewing it.

I'm not sure and familiar with... Should I add your 'reviewed by' into
my patches by myself?

> 
> > Mind fixing rt.c if it hasn't been fixed already.
> > 
> > -- Steve